1. Field of the Invention
The present invention relates to an optical scanning device.
2. Description of the Related Art
In color image forming apparatuses such as color laser printers, different color information is written with scanning beams of laser beams by multiple optical scanning units (optical scanning device) with scanning-image-forming optical systems independently to multiple photoconductors that are rotated by a driving mechanism, respectively. A tandem color image forming apparatus has been available in which by such writing with scanning beams, electrostatic latent images are formed, and these electrostatic latent images are visualized by multiple visualizing units into visible images in different colors, respectively, and then transferred onto a transfer material overlapped with each other, thereby obtaining a color image.
The respective optical scanning units described above emit laser beams from a laser constituted of a semiconductor that is driven and controlled according to a read image information signal of each color. The laser beam is concentrated on a surface of the photoconductor that is uniformed charged through optical parts, such as a polygon mirror and a lens, and scanned in a main scanning direction. On the surface of the rotating photoconductor, an image signal corresponding to multiple scanning beams are written as scanning beams at predetermined intervals, and an electrostatic latent image is formed thereon.
In such an image forming apparatus, by misregistration in a sub-scanning direction on a surface of the photoconductor caused by vibration of the optical scanning device from inclination of the surface of a polygon mirror and rotation of the polygon mirror, banding (density unevenness in the sub-scanning direction, horizontal strips) in a small pitch of about 1 millimeter (mm) to several mm in a cycle of a single rotation component of the polygon mirror occurs.
For the banding described above that is one of the points focused on in image quality, various techniques to suppress this phenomenon have been studied.
Particularly, as for the surface inclination of polygon mirrors, the precision in processing mirrors have almost reached the limit, and countermeasures other than that in the polygon processing have been demanded.
Near the surface of the respective photoconductors, a non-contact displacement meter to detect a displacement amount relative to an X-axis direction, and a non-contact displacement meter to detect a displacement amount relative to a Z-axis direction are arranged.
To each cylindrical lens in the optical scanning device, a piezoelectric element to shift in the Z-axis direction is mounted. Each optical detector sensor outputs a synchronization detecting signal, a light-amount monitor signal, and a sub-scanning displacement signal for a corresponding photoconductor drum.
A scanning control device controls the piezoelectric element for each photoconductor drum, based on a result of combining the output of the non-contact displacement meters and the sub-scanning displacement signal.
With the above configuration, a sub-scanning displacement is variably adjusted by minutely shifting the cylindrical lens in the Z-axis direction according to banding, and banding can be thereby suppressed.
However, in the above method, a structure to make a sub-scanning position variable by causing mechanical vibration to a cylindrical lens of by a piezoelectric element is required. Therefore, there is a scope for improvement in size and cost for a high-voltage power-supply circuit for piezoelectric driving and an optical housing.
In view of the above, there is a need to provide an optical scanning device that enables banding (density unevenness in a sub-scanning direction) to be suppressed by adjusting a laser beam position in the sub-scanning direction, and that achieves a high image quality and stabilizes an image quality at low cost.